1. Field of the Invention
This invention relates generally to differential pressure detectors and more specifically to such a detector utilizing permanent magnet means with the magnetic fields thereof interacting to provide a signal representative of predetermined differential pressure conditions present in a filter.
2. The Prior Art
It is common in the prior art to provide differential pressure sensing devices which provide a signal such as the extension of a button or the like when the differential pressure across a filter exceeds a certain preset level. The differential pressure reaching the preset level is indicative of the filter being loaded with contaminants and in need of cleaning or replacing. Such devices commonly utilize permanent magnets which are positioned in line within the apparatus such that when the differential pressure present across a piston moves one of the magnets carried by the piston away from the other, the attracting forces between opposite poles of the two magnets is weakened to a degree such that a spring can move the other magnet and, responsive thereto, provide the signal indicating the differential pressure threshold having been reached. Typical of such devices is that disclosed in U.S. Pat. Nos. 2,942,572; 3,077,176; 3,077,854; 3,140,690 and 3,786,332. Typical of such prior art devices is that upon the predetermined differential pressure being reached a bypass valve opens allowing direct flow of unfiltered fluid from the source to the load. Such unfiltered fluid may contain unwanted contaminants that could affect operation of the load.
Under some applications it is desirable to provide a device which will signal the user that the differential pressure of concern has not as yet been reached but that it is impending. Such devices would allow replacement of the filter prior to bypass occurring. Devices of the type shown in the prior art such as that in the above-referred to patents provide only a go/no-go indication and do not provide a dual indication functioning as a caution to the user.